1. Field of the Invention
The invention relates to a thermal printhead for a thermal printer comprising an electrically insulating substrate on which resistors are placed that form impression points, as well as current supply leads and current discharge leads bonding the resistors.
2. Description of the Prior Art
West German Patent Publication No. 25 37 142 discloses a thin-film thermal printhead with a substrate, a glass layer on the substrate, heating elements of high-resistivity material, a plurality of electrical conductors connected to the heating elements for the supply of electrical power, and a top layer made of wear-resistant material with a relatively high heat conductivity. Gold or aluminum is used as conducting material for producing the electric conducting paths for supplying current to, and discharging it from, the single resistor impression points. This material for the electric conducting paths, is, for example, deposited by evaporation or by cathode sputtering and bonded with the single resistor.
However, since the height of the gold layer is considerably less than the height of the single resistors, current distribution during operation occurs only on the level of the gold layer within the resistors so that, due to the resultant heat distribution, the upper edges and the upper portion of the resistor are not heated sufficiently. Furthermore, the metal conducting layer dissipates the heat very quickly. These disadvatages lead to an unsatisfactory heat distribution within the resistor, so that more current is required for the heating thereof. However, this advantage persists even with more current in that the greatest current density is within the lower part of the resistor, i.e., on the level of the bonding. Therefore, the highest temperature prevails at the foot of the resistor between the bonding of both conducting paths instead of in the upper part of the resistor as is required.
It should be noted that the term "resistor" is synonymous with the terms "resistance element" and "heating element."